Go to your sound control panel, recording tab.
If you don't see stereo mix there, right click in the white space of the tab and make sure that "show disabled devices" and "show disconnected devices" are both checked.
If stereo mix does not then show up, you have two choices:
1: Try any and all drivers for your sound card/chip (Vista, Windows 7, whatever). Install them in any and all possible modes (normal way, compatibility mode, etc)
2: Buy a sound card KNOWN to provide stereo mix on Windows 7. Many cards WILL NOT provide stereo mix regardless of driver on Windows 7. That's just the way it is.
Since you have a laptop, you may have to use an external USB sound card if you can't find a driver that will work.